[feat] Add new decorator assign_units for unit processing on input and output for a function that does not support unit operations

[Routhleck]

This pull request introduces a new decorator function assign_units in the brainunit module to handle unit transformations for function arguments and return values. It also includes tests for this new functionality. The most important changes include adding the assign_units function, updating the _base.py file to include the new decorator, and adding corresponding tests.

New functionality for unit transformation:

• Added assign_units decorator function to transform units of arguments passed to a function. (brainunit/_base.py)

Codebase updates:

• Included assign_units in the list of functions for checking units. (brainunit/_base.py)
• Added _assign_unit helper function to handle unit assignment within the assign_units decorator. (brainunit/_base.py)

Testing:

• Added test_assign_units method to test various scenarios for the assign_units decorator, ensuring it correctly handles unit transformations and raises appropriate errors. (brainunit/_base_test.py)

[Routhleck]

#72

[chaoming0625]

Great.